The control of the evaporation loss emanating from the liquids stored in the tanks is effected, in the prior art, through the utilization of floating roofs.
These floating roofs consist essentially of a metallic pan that floats on the surface of the stored liquid.
Sealing means are provided to close the annular rim space between the periphery of the pan and the relatively concentrical tank shell. Two general types of seal are prominently used in the prior art.
For purposes of description these seals can be classed as metallic and non-metallic. The metallic class utilizes a light gage metal band as the sliding contact with the tank shell. This band is formed by sheets (called shoes) and may vary in dimension from the different manufacturers. The shoes are joined together to form a complete ring.
Generally, the band is of galvanized steel approximately 16 gage. Provision is made for expansion or contraction of the ring as it passes over shell irregularities, rivets and the like. This is accomplished by narrow pieces of fabric joined into the ring. The rim space between the roof and the shoe is sealed by bolting or clamping a coated-fabric from the shoe to the rim plate of the roof.
The non-metallic class utilizes a coated-fabric band as the sliding contact with the tank's shell. The forces needed to expand the seal against the tank shell are provided by a liquid, a gas under pressure, or a resilient foam.
The tank sizes to which the present invention is directed vary from about 20 feet up to about 250 or 300 feet in diameter.
Perfect fit between the seals and the tank shell is a practical impossibility for such a large structures, mainly due to out of roundness and local irregularities in the shell.
Evaporation of the stored liquid takes place through the gaps between the seal and the shell.
In practice this evaporative loss attains sizable proportions due to the substantial length of the sealed perimeter.
The present invention, by providing a tight fit over the whole perimeter, overcomes the shortcomings of the prior art.